The researchers analyzed data from 12 studies with a total of 518,520 participants to assess the stroke risk of slightly elevated blood pressure, also known as prehypertension. The review was released online Wednesday in the journal Neurology.

Almost one-third of the U.S. population is believed to have prehypertension, defined as systolic pressure between 120 and 139 and diastolic pressure between 80 and 89. Normal blood pressure is a systolic reading of less than 120, and a diastolic measure of less than 80.

In general, the studies revealed that slightly elevated blood pressure was linked with a 50% higher risk for stroke compared with people with normal blood pressure, even after controlling for variables such as obesity, diabetes, smoking and age.

In the seven studies that divided slightly high blood pressure into a low range (systolic pressure between 120 and 129 and diastolic pressure between 80 and 84) and a high range, people in the low range didn't show substantially higher-than-normal stroke risk. However, for those in the high range the risk for stroke was much higher than normal.

Although stroke is often associated with advanced age, this research review showed that people under age 65 who had slightly high blood pressure were almost 80% more apt to have a stroke versus people with normal blood pressure.

This younger at-risk population might gain from lifestyle changes, the authors wrote.

"These people may immediately benefit from blood pressure lowering methods, such as reducing their salt intake and weight, to help reduce their risk of stroke," co-author Dr. Bruce Ovbiagele of UC San Diego, said in a news release. He added that more studies are needed to see whether efforts to lower pressure can reduce the chances of having a stroke.